linux-ext4 - [RFC 7/7] ext4: Support atomic write for statx

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Date: Thu, 30 Nov 2023 19:23:16 +0530
From: Ojaswin Mujoo <ojaswin@...ux.ibm.com>
To: linux-ext4@...r.kernel.org, "Theodore Ts'o" <tytso@....edu>
Cc: Ritesh Harjani <ritesh.list@...il.com>, linux-kernel@...r.kernel.org,
        "Darrick J . Wong" <djwong@...nel.org>, linux-block@...r.kernel.org,
        linux-xfs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        John Garry <john.g.garry@...cle.com>, dchinner@...hat.com
Subject: [RFC 7/7] ext4: Support atomic write for statx

Support providing info on atomic write unit min and max for an inode.

For simplicity, currently we limit the min at the FS block size, but a
lower limit could be supported in future.

Signed-off-by: Ojaswin Mujoo <ojaswin@...ux.ibm.com>
---
 fs/ext4/inode.c | 38 ++++++++++++++++++++++++++++++++++++--
 1 file changed, 36 insertions(+), 2 deletions(-)

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index d185ec54ffa3..c8f974d0f113 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -5621,6 +5621,7 @@ int ext4_getattr(struct mnt_idmap *idmap, const struct path *path,
 	struct ext4_inode *raw_inode;
 	struct ext4_inode_info *ei = EXT4_I(inode);
 	unsigned int flags;
+	struct block_device *bdev = inode->i_sb->s_bdev;
 
 	if ((request_mask & STATX_BTIME) &&
 	    EXT4_FITS_IN_INODE(raw_inode, ei, i_crtime)) {
@@ -5639,8 +5640,6 @@ int ext4_getattr(struct mnt_idmap *idmap, const struct path *path,
 
 		stat->result_mask |= STATX_DIOALIGN;
 		if (dio_align == 1) {
-			struct block_device *bdev = inode->i_sb->s_bdev;
-
 			/* iomap defaults */
 			stat->dio_mem_align = bdev_dma_alignment(bdev) + 1;
 			stat->dio_offset_align = bdev_logical_block_size(bdev);
@@ -5650,6 +5649,41 @@ int ext4_getattr(struct mnt_idmap *idmap, const struct path *path,
 		}
 	}
 
+	if ((request_mask & STATX_WRITE_ATOMIC)) {
+		unsigned int awumin, awumax;
+		unsigned int blocksize = 1 << inode->i_blkbits;
+
+		awumin = queue_atomic_write_unit_min_bytes(bdev->bd_queue);
+		awumax = queue_atomic_write_unit_max_bytes(bdev->bd_queue);
+
+		if (!(ext4_test_inode_flag(inode, EXT4_INODE_EXTENTS)) ||
+		    EXT4_SB(inode->i_sb)->s_cluster_ratio > 1) {
+			/*
+			 * Currently not supported for non extent files or
+			 * with bigalloc
+			 */
+			stat->atomic_write_unit_min = 0;
+			stat->atomic_write_unit_max = 0;
+		} else if (awumin && awumax) {
+			/*
+			 * For now we support atomic writes which are
+			 * at least block size bytes. If that exceeds the
+			 * max atomic unit, then don't advertise support
+			 */
+			stat->atomic_write_unit_min = max(awumin, blocksize);
+
+			if (awumax < stat->atomic_write_unit_min) {
+				stat->atomic_write_unit_min = 0;
+				stat->atomic_write_unit_max = 0;
+			} else {
+				stat->atomic_write_unit_max = awumax;
+				stat->attributes |= STATX_ATTR_WRITE_ATOMIC;
+			}
+		}
+		stat->attributes_mask |= STATX_ATTR_WRITE_ATOMIC;
+		stat->result_mask |= STATX_WRITE_ATOMIC;
+	}
+
 	flags = ei->i_flags & EXT4_FL_USER_VISIBLE;
 	if (flags & EXT4_APPEND_FL)
 		stat->attributes |= STATX_ATTR_APPEND;
-- 
2.39.3